Plants in which a liquid medium passes through a plurality of thermal systems in order to be heated, possibly evaporated, are present, for example, in boilers which are heated by flue gas from burners or exhaust gas from gas turbines.
The medium may be water, having additives if need be. Depending on the final boiler load, the water is heated in the boiler to a predetermined temperature in order to be fed, for example, to an industrial plant, a hot-water network, etc., or evaporated in order to be fed, for example, to a steam turbine or an industrial steam load.
The first thermal system in the boiler of such a plant is normally called an economizer, and may include a first heat exchanger and a heating-area bank. Due to temperature conditions, the economizer, which is provided for the cooling of the flue gas and preheating feed-water to be introduced into the boiler by a boiler inlet, preferably works on the flue-gas-side or exhaust-gas-side end of the boiler, e.g., at comparatively low temperatures when compared to the temperatures in the boiler itself.
On the other hand, the temperature difference between the flue gas or exhaust gas and the feed-water to be heated is relatively small. This in turn results in large heating areas and large heating-area masses associated therewith. Furthermore, it is known that there is a risk of dew-point corrosion on account of the temperatures and pressures prevailing in the economizer.
Known methods of raising the feed-water temperature at the boiler inlet and for avoiding dew point corrosion within the economizer include recirculation wherein water preheated by the boiler is admixed with the feed-water. Power plants utilizing recirculation may do so throughout all of the various operating loads under which they operate, or they may selectively recirculate the feed-water so that recirculation is only utilized at start-up and/or low operating loads.
A power plant utilizing recirculation may include a pumped start-up system used at start-up and at low operating loads, e.g., conditions where the feedwater flow is not of sufficient quantity to protect the waterwall tubes from overheating due to the combustion of fuel taking place in the boiler furnace. Such a power plant may include a main bypass line that diverts incoming feed-water from a main feed-water line to a mixing device wherein the feed-water is mixed with recirculated water previously heated by the boiler. The recirculated water heats the feed-water in the mixing device and then the mixed feed-water is pumped to an economizer feed-water line downstream of the bypass line and is eventually supplied to the economizer. The mixing device must be relatively large in order to handle a flow rate of 30% to 40% of full operational load.
Once the power plant reaches a particular operating load, the feedwater flow is of sufficient quantity to protect the waterwall tubes from overheating and exhaust gas temperatures increase to a point where the economizer may operate optimally without pre-heating the feed-water by recirculation. When the power plant reaches such operating conditions, the flow of feed-water to the main bypass line is stopped. The power plant may then operate in a once-through mode wherein feed-water is not recirculated.
When the power plant is in the recirculation mode, the mixing device must mix the saturated, recirculated water with the relatively cold feed-water without generating excessive thermal stress in the mixing device or in subsequent components downstream of the mixing device. The mixing device must also contain a mechanism for preventing debris from reaching the downstream components of the power plant, particularly a circulation pump used for pumping the mixed feed-water back to the main feed-water line.
Typically, the mixing of the saturated recirculated water with the relatively cold feed-water is performed in a drum-type unit having sleeved nozzles. In once through boilers the mixing process is accomplished by a mixing tee. The mixing tee includes an outer pipe having a first diameter for transporting the cold feed-water and an inner pipe having a second smaller diameter for transporting the saturated, recirculated water. The inner pipe contains a series of holes around its circumference and along its length to allow for mixing of the two liquids.
However, the mixing tee has several drawbacks. Firstly, the inner pipe is inaccessible for inspection, cleaning or repair. Thus, if a defect is suspected, the entire assembly must be disassembled to inspect, thereby causing an increase in plant downtime for maintenance. Secondly, the mixing tee is difficult to construct and install; the relatively small spacing between the pipes leaves little room for error and is relatively complex to assemble. Therefore, construction costs are increased and replacement of the mixing tee is a complicated procedure leading to additional plant downtime. In addition, the mixing tee must be used in conjunction with a sieve for debris removal. The sieve is a complex combination of perforated plates and screens, and typically requires a pressure seal cover which is expensive, difficult to maintain and prone to scoring and leaks. Furthermore, the mixing tee and sieve are formed as two separate pressure parts.
What is needed is a mixing device which combines mixing and filtering elements in a single pressure part and which is easy to construct, install, inspect, maintain and replace.